The so-called thin film resistor, which has a metal thin film resistive layer formed on an insulated support, wherein the layer is formed by vacuum deposition, cathode sputtering, or plating, provides a better properties than known metalglazed resistor or printed resistor in terms of temperature coefficient of resistance, sliding noise, and load life. However, the thin film resistor has a very short slide life and hence it has limited utility in a slide variable resistor.
The substrate-type thin film resistor, which is obtained by subjecting to a subtractive method treatment (i.e. mask etching method), a three-layer printed circuit board having a metal thin film resistive layer deposited, for example, by plating on one side of a copper foil, and has an insulated support superposed thereon, is disclosed in U.S. Pat. No. 4,220,945. This resistor is better in terms of temperature coefficient of resistance, uniformity of resistance, and other similar properties. Further, this resistor has advantages such as being light in weight, it is free of cracking and chipping, and its production cost is low. Nevertheless, the metal thin film resistive layer in this resistor is very vulnerable to mechanical friction. Hence, the resistance value offered by the resistor is readily increased as the sliding member is slid on the surface of the metal thin film resistive layer.
Furthermore, this resistive layer, without a cover coat of insulating resin, is deficient in other properties such as resistance to moisture, resistance to heat, and resistance to soldering. Thus, the substrate type thin film resistor is not useful as a resisting member in a slide variable resistor.
A resistor having a multiplicity of gold-plated electrodes arranged in the pattern of teeth of a comb on a metal thin film resistive layer pattern and having a slide member adapted to be slid along the array of electrodes has also been used. However, this resistor is not suitable when fine adjustments of the resistance value are required since the variation in the resistance value effected by the movement of the sliding member occurs stepwise and not continuously.
Furthermore, this resistor has a disadvantage in that it requires incorporation of an electrode portion adapted for the sliding member to be slid thereon and, consequently, necessitates a proportionate addition to the overall width of the resistor.
In addition, electrodes having the pattern of teeth of a comb are disadvantageous since the gold plate deposited on the electrodes is gradually abraded and scraped off in the form of fine particles as the number of movements of the sliding member thereon increases. Eventually, this results in the individual electrodes being short-circuited.